Which statement about life-table assumptions is true?

• A. Survivorship is assumed to improve over calendar time.
• B. They assume survivorship varies randomly with time.
• C. They assume no changes in survivorship over calendar time, and those lost to follow-up have the same survivorship as those who are followed.
• D. They assume everyone has the same mortality rate regardless of age.

Answer: (C)

Life-table assumptions in survival analysis require that survivorship does not change with calendar time and that loss to follow-up is non-informative—those who are censored have the same survival prospects as those who remain observed. This is why the statement is true: it captures these two standard ideas. If survivorship varied over time, or if those who drop out had different survival, the estimates would be biased. Also, life tables typically involve age-specific mortality, so assuming the same mortality rate for everyone regardless of age would miss the fundamental age pattern in mortality.